Heading to Oxford for a few days, go to Bath at the start or end of trip?

Hi folks, going to Oxford for a four days in early May for work, first time in England

Will most likely arrive in the morning on Day 1, and leave at night on Day 4 from Heathrow.

I don't plan to span any time in London, but i do want to visit Bath for a day trip. The free time I have will be on Day 1 and Day 4.

When would be the best day to head to Bath? is it more peaceful for walks on a weekday or on a weekend?

Also I plan to buy some hard cheese to bring back, will it survive the weather in my bag in May or best to buy just before i make it to Heathrow?